Lithium battery labels usually show voltage, amp-hour capacity and sometimes total energy in kilowatt-hours.
Amp-hours alone do not tell the full story because the same Ah rating at different voltages represents different amounts of stored energy.
A useful comparison is energy in kilowatt-hours. For example, a 51.2V 100Ah battery is roughly 5.12kWh before considering usable depth of discharge and system losses.
Your actual backup time still depends on the load connected to the battery. A 5kWh battery powering a light load will last much longer than the same battery powering several heavy appliances.
When comparing batteries, also consider the battery management system, recommended charge and discharge limits, warranty terms and compatibility with your inverter.